The present invention relates generally to micro-electromechanical systems (xe2x80x9cMEMSxe2x80x9d) devices. More particularly, the present invention relates to a MEMS device that is movable between a first position located within a multi-layer substrate and a second position that is located outside of the substrate.
MEMS technology is becoming ubiquitous. MEMS accelerometers, pressure sensors, and even MEMS-based electrical components have been developed for use in a wide variety of applications.
Presently, some of the most important applications for MEMS are in the area of optical communications, wherein MEMS-based optical modulators, switches, attenuators, filters and like devices have been developed. While MEMS technology is very well suited for optical communications applications, integrating MEMS devices into such systems does present certain challenges. In particular, to process an optical signal via a MEMS device, the MEMS device must typically capture or engage the optical signal in a region of space that is xe2x80x9cout-of-the-planexe2x80x9d relative to the substrate layer of the MEMS device. In other words, a raised or three-dimensional MEMS component is required to capture the signal.
Such components have traditionally been fabricated as xe2x80x9cflip-upxe2x80x9d structures that incorporate micro-hinges. Flip-up structures are formed by (1) fabricating hinged plates that lie on a base or substrate layer; (2) raising the hinged plates by rotating them about their micro-hinges; and (3) locking the hinged plates into the raised position. See, e.g., U.S. Pat. Nos. 5,923,798 and 5,963,367; Pister et al., xe2x80x9cMicrofabricated Hinges,xe2x80x9d Sensors and Actuators A, vol. 33, pp 249-256 (June 1992); Lee et al., xe2x80x9cSurface-Micromachined Free-Space Fiber Optic Switches with Integrated Microactuators for Optical Fiber Communication Sytsems,xe2x80x9d Transducers ""97, 1997 Int""l. Conf. Solid-State Sensors and Actuators, Chicago, Jun. 16-19, 1997, pp 85-88, and Reid et al., xe2x80x9cAutomated Assembly of Flip-Up Micromirrors,xe2x80x9d Transducers ""97, 1997 Int""l. Conf. Solid-State Sensors and Actuators, Chicago, Jun. 16-19, 1997, pp 347-350.
While flip-up optical MEMS structures represent a tremendous advance over earlier bulk devices having moving parts, they nevertheless suffer from certain drawbacks.
For instance, many of the MEMS foundries offer fabrication processes that use alternating layers of oxide and polysilicon to form the various plates and other elements of a MEMS structure. Typically, the polysilicon layers exhibit compressive stress that can cause the fabricated elements to warp. Warped elements can cause assembly and operational problems. Additionally, flip-up structures must be assembled. In some cases, processing steps and structures are required for no reason other than to drive the assembly process.
Furthermore, optical applications often have stringent placement tolerances (e.g., for single mode fiber, etc.). Due to the nature of (i.e., the xe2x80x9cplayxe2x80x9d in) micro-hinges, rotating a plate or other hinged element into a precise position is problematic. Moreover, once a hinged element is moved to a desired position, it must be locked in place. The locking mechanism is often realized as an additional notched plate that is rotated into interlocking engagement with the hinged element. Again, the notched plate represents additional fabrication and assembly steps.
The art would therefore benefit from an article that offers the functionality of the flip-up structures of the prior art but avoids at least some of their drawbacks.
The present invention is directed to a MEMS device that avoids some of the drawbacks of the prior art. The present MEMS device, which is fabricated from a multi-layer substrate, comprises a support portion coupled to an element portion. In some embodiments, the MEMS device is configured as an optical switching element, wherein, for example, the element portion is physically adapted to receive and reflect an optical signal. It is to be understood, however, that such embodiments are merely illustrative; in other embodiments, the present MEMS device is suitably used as other than an optical switching element.
The multi-layer substrate that forms the present MEMS device has at least a first layer, a second layer and an intermediate layer that separates the first and second layers. In accordance with the present teachings, the element portion of the MEMS device is fabricated from the second layer of the multi-layer substrate. Furthermore, it is particularly advantageous if the element portion has a major surface that is defined by the thickness of the second layer of the multi-layer substrate. When formed this fashion, the element portion itself is substantially orthogonal to the major surface of the surrounding multi-layer substrate.
In one embodiment, the multi-layer substrate is a silicon-on-insulator wafer. The support portion is advantageously formed from the top, relatively thin silicon layer of the silicon-on-insulator insulator wafer, while the element portion is formed from the bottom, relatively thick silicon layer of the wafer.
In some embodiments, the support portion of the MEMS device includes an actuating plate, one or more torsional members and a beam. The beam advantageously rigidly couples the actuating plate to the element portion. The torsional members, which depend from the beam, are coupled to the first layer of the multi-layer substrate (from which the torsional members are formed). The MEMS device is therefore supported, via the torsional members, from the multi-layer substrate. The torsional members are operative to twist, thereby allowing the MEMS device to move (e.g., rotate, etc.) independently of the multi-layer substrate.
The actuating plate and the element portion, which depend from the beam, are disposed on opposite sides of an axis of rotation that is aligned with the torsional members (i.e., the axis of rotation of the beam). Furthermore, the actuating plate is advantageously rigidly coupled to the element portion. This configuration functions as a mechanism by which at least some of the element portion formed from the second layer is raised xe2x80x9cabovexe2x80x9d the first layer of the substrate.
Specifically, in a first position, the element portion is disposed within the multi-layer substrate. In some embodiments, the first position results when the MEMS device is in an unactuated state (i.e., no potential difference between the actuating plate and the underlying electrode). In an actuated state, a potential difference is created across the actuating plate and the electrode, thereby generating an electrostatic force of attraction therebetween. When the electrostatic force exceeds the restoring force of the torsional members, the actuating plate moves toward the underlying electrode such that the beam rotates about its axis of rotation. As the actuating plate moves downwardly toward the electrode, the element portion moves upwardly out of the multi-layer substrate to a second position in seesaw-like fashion. When the actuating force is removed, the element portion drops back within the multi-layer substrate to the first position.
The term xe2x80x9crestoring force,xe2x80x9d as used herein, is the force of the torsional elements that must be overcome in order to move the element portion from its unactuated state to its actuated state. The term xe2x80x9cunactuated state,xe2x80x9d or xe2x80x9cfirst positionxe2x80x9d as used herein, refers to when the element portion is within (i.e., beneath the surface of) the multi-layer substrate. The term xe2x80x9cactuated state,xe2x80x9d or xe2x80x9csecond position,xe2x80x9d as used herein, refers to when the element portion is outside (i.e., above the surface) of the multi-layer substrate.
In the context of an optical switching element, the element portion is used to direct an optical signal. For example, in one embodiment, the two optical fibers are positioned end-to-end, with a gap between the ends, over the multi-layer substrate. The element portion of the present MEMS device is aligned with the gap between the fiber ends. When the MEMS device is in the first position within the multi-layer substrate, the optical signal is able to pass from the first fiber to the second fiber. When, however, the MEMS device is actuated, the optical signal does not pass from the first fiber to the second fiber since the element portion is raised above the surface of the multi-layer substrate and into the path of the optical signal.
In such embodiments, the element portion is configured and placed to direct the optical signal either back to the input source of the optical signal (i.e., the input fiber or input waveguide) or to a different fiber or waveguide. Thus, the optical switching element is used to either redirect or reflect the optical signal.